Technical Field:
This invention relates to an hydraulically operated double acting slip assembly useful to grip pipe as required while pipe is being run into or pulled from a well.
2. Related Art:
This invention is an improvment of my slip assembly of U.S. Pat. No. 4,576,254, which is herein incorporated for reference.
My previously patented slip assembly structure included double acting slip systems, which for being operated to grip pipe, automatically gripped the pipe tigher on slight upward or downward movement of the pipe caused by a small upward or downward load on the pipe. These small loads may occur each time the slips are operated to grip pipe as pipe is being run into or pulled from a well through the slip assembly. Slight movement of the pipe automatically causes the slip systems to grip pipe tighter and forces resulting from the tighter grip induce frictional forces between slip assembly members, which are greater than forces applied to the slip assembly members by a pressurized hydraulic operating cylinder to move the slip systems to a position not gripping pipe; therefore, the slip systems cannot be moved inadvertently or on purpose to a position not gripping pipe unless the pipe is repositioned slightly upwardly or downwardly to the precise "neutral" position where the automatic grip tightening system is not operating.
Field operation of the slip assembly of the previously mentioned U.S. patent showed a need to increase the height of the neutral position "band" as operators had to vertically reposition pipe small distances many times, when the slip systems were automatically gripping pipe tighter, before the pipe was at the precise level where the pressurized operating cylinder could move the slip systems to a position not gripping pipe. It is very difficult and time-consuming to move heavy pipe loads repeatedly up or down small distances trying to locate a very narrow neutral position band. Also, manufacturing problems and expense experienced with the structure used to position and guide the wedges and slip systems required new structure.